Research Question

* Can we develop a predictive procedure of bank erosion in river channels and reservoirs to improve the design of river restoration strategies and the management of reservoir sediment?

Need and Benefit

There is a critical need for the Bureau of Reclamation (Reclamation) to understand and predict bank erosion and river meandering processes because these processes directly affect Reclamation operations and maintenance. Commonly applied one-dimensional (1D) models (such as HEC-RAS) are insufficient to model the bank erosion or meandering process because they cannot resolve the complicated flow that occurs in rivers. In fact, most 1D models do not even attempt to model the process. A two-dimensional (2D) hydraulic and sediment transport model (SRH-2D) was previously developed, and it has been extensively used for many projects. We will build upon this tool and add the U.S. Department of Agriculture (USDA) Agricultural Research Service (ARS) bank erosion module (BSTEM) to it. There are no existing 2D models of bank erosion commercially available that have the required flexibility and accuracy.

There are several reasons why the new SRH2D-BSTEM model will benefit Reclamation:

* Bank erosion is a critical process in the erosion of sediment in reservoirs resulting from dam removal or from the sluicing of sediment from reservoirs. The SRH-2D-BSTEM model could be used to predict the erosion of sediment following dam removal or during reservoir sluicing. It has already been used at Robles Diversion Dam on the Ventura River and Palo Verde Dam on the Colorado River to predict various sediment sluicing strategies. However, the model was limited in that it could not predict bank erosion and, therefore, some of the quantitative predictions of the amount of sediment sluiced were not accurate. Several upcoming dam removal projects will benefit from the better prediction of bank erosion processes, including the proposed removal of four PacifiCorp dams on the Klamath River and Matilija Dam on the Ventura River.

* Bank erosion is important to the design and sustainability of fish habitat. Several restoration projects, such as the Trinity River Restoration Project, San Joaquin Restoration Project, and the restoration projects on the tributaries to the Columbia Basin, identify off-channel, side-channel, or the channel margins as habitat important to the success of juvenile anadromous fish. Often, this off-channel habitat has been reduced because of the reduction in peak flows and sediment after dam construction. A critical unknown in many river restoration strategies is their effects on bank erosion and planform of the river channel. A numerical model could be used to assess various strategies and choose the one with the greatest positive effect.

* Bank erosion is important to the sustainability of riparian habitats. Most riparian communities in the western U.S. depend upon an active channel that creates reworked bare sediment surfaces that can be repopulated by native woody species. If there is no active channel, the riparian forests are not regenerated, and new generations of woody species are not created. In addition, the amount of woody structure in the river may be decreased. A numerical model could be used to assess strategies to increase bank erosion at select sites.

* Bank erosion is important to the design of bank protection measures. Diversions, bridges, pumping plants, levees, and other structures located along the river often require engineered bank protection measures. However, there are environmental concerns with many of these bank protection measures, and they need to be applied selectively. A numerical model can be used to assess where bank protection is required and find other alternatives to hard bank protection. The model can also be used to assess the effect of the bank protection on the upstream and downstream reaches.

Hence, this research will develop a practical tool that can help predict the impact of sediment releases, as well as assist in the design of sediment sluicing projects, river restoration projects, and bank protection projects.

Contributing Partners

None

Research Products

Independent Peer Review

The following documents were reviewed by qualified Bureau of Reclamation employees. The findings were determined to be achieved using valid means.

This study shows that improved predictions are obtained with the present model relative to previous model investigations. The study also shows that the current model is adequate for predicting contraction scour and model results are comparable with those of three-dimensional modelling except for the prediction of aggradation downstream of the contracted channel section.

Keywords: two-dimensional model, scour

Bureau of Reclamation Review

The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.

Based on a literature review, we proposed to develop the current state-of-the-art bank erosion capability into the existing mobile-bed model SRH-2D. The non-coheisve bank erosion algorithm is tested against laboratory data.

In this study, a two-dimensional model (SRH-2D) is coupled with a deterministic bank stability and toe erosion model (BSTEM) to predict channel adjustment and planform development. Herein, the proposed coupling approach is described, along with numerical aspects of the procedures. For test and verification purposes, the coupled model is used to predict bank retreat of Goodwin Creek in Mississippi. A comparison of the model results with the measured data is presented and discussed.

A two-dimensional (2D), depth-averaged, mobile-bed model was developed to predict the scour at the eroding bankline. Scour depths estimated by the 2D mobile-bed model scenarios compare reasonably with the scour depths estimated from empirical equations. The 2D mobile-bed model also identifies locations of maximum scour depth, which, when compared with field observations of the largest scour depth location, provided insight that helped refine and narrow the realistic scour depth range.